Asteroid 2015 CO13 passed by the Earth at a distance of 5 145 000 km (13.4 times the average distance between the Earth and the Moon, or 3.44% of the average distance between the Earth and the Sun), at about 7.30 pm GMT on Tuesday 24 February 2015. There was no danger of the asteroid hitting us, though had it done so it would have presented minor threat. 2015 CO13 has an estimated equivalent diameter of 30-94 m (i.e. it is estimated that a spherical object with the same volume would be 30-94 m in diameter), and an object towards the upper end of this range would pass through the atmosphere and directly impact the ground with a force of about 30 megatons of TNT (about 1800 times the explosive energy of the Hiroshima bomb), causing devastation over a wide area and creating a crater about kilometer across.
The calculated orbit of 2015 CO13. JPL Small Body Database.
2015 CO13 was discovered on 13 February 2015 (Eleven days before its closest approach to the Earth) by the University of Arizona's Mt. Lemmon Survey at the Steward Observatory on Mount Lemmon in the Catalina Mountains north of Tucson. The designation 2015 CO13 implies that it was the 339th asteroid (asteroid O13) discovered in the first half of February 2015 (period 2015 C).
2015 CO13 has a 1033 day year orbital period and an eccentric orbit tilted at an angle of 3.13° to the plane of the Solar System, which takes it from 0.65 AU from the Sun (i.e. 65% of the average distance at which the Earth orbits the Sun, slightly inside the orbit of Venus) to 3.35 AU from the Sun (i.e. 335% of the average distance at which the Earth orbits the Sun, considerably more than twice the distance at which the planet Mars orbits the Sun). It is therefore classed as an Apollo Group Asteroid (an asteroid that is on average further from the Sun than the Earth, but which does get closer).
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